We have cloned a full-length copy of double stranded DNA that codes for an influenza virus polymerase protein, PB2. PB2 is one of the two basic proteins that are present in small copy number in the viral nucleocapsid. PB2 binds and then cleaves capped host cell mRNA. This represents the first step in the priming of influenza viral mRNA transcription. We have initiated attempts express functional PB2 in mammalian cells. Initially, cloned PB2 DNA flanked by Bam HI linker sequences was inserted into the late region between the Hpa II and the Bam HI sites of an SV40 vector that contains a viable deletion in the small t region; this deletion provides additional room for packaging foreign DNA in SV40. Recombinant PB2-SV40 DNA was used for transfection of primate cells and was successfully propagated in the presence of an early SV40 ts mutant helper. Synthesis of the PB2 polypeptide in recombinant infected cells is currently being analyzed by immunoprecipitation and by in vitro translation of PB2 specific mRNA. A recombinant that expresses polymerase PB2 should be useful for determining whether PB2 by itself exhibits the functional activity that has been ascribed to it as a coponent of the nucleocapsid transcriptase complex. In addition, complementation analysis of ts influenza mutants will be carried out to test the biologic activity of PB2 produced in recombinant infected cells.